Congressional report worries over Falcon 9 engine cracks

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A forthcoming congressional report, reported by the Wall Street Journal, reveals that NASA is concerned about cracks that occur in the turbopumps of SpaceX’s Merlin engines.

The newspaper says the report has found a “pattern of problems” with the turbine blades within the turbopumps, which deliver rocket fuel into the combustion chamber of the Merlin rocket engine. Some of the components used in the turbopumps are prone to cracks, the government investigators say, and may require a redesign before NASA allows the Falcon 9 booster to be used for crewed flights. NASA has been briefed on the report’s findings, and the agency’s acting administrator, Robert Lightfoot, told the newspaper that he thinks “we know how to fix them.”

A spokesman for SpaceX, John Taylor, said the company already has a plan in place to fix the potential cracking issue. “We have qualified our engines to be robust to turbine wheel cracks,” Taylor said. “However, we are modifying the design to avoid them altogether. This will be part of the final design iteration on Falcon 9.” This final variant of the Falcon 9 booster, named Block 5, is being designed for optimal safety and easier return for potential reuse. According to company founder Elon Musk, it could fly by the end of this year.

Here’s the real scoop: SpaceX initially built the engines to fly once, just as every single rocket company has done in the entire history of space, excluding the space shuttle. Under these conditions, the cracks could be considered an acceptable issue, which is what they mean when they say “We have qualified our engines to be robust to turbine wheel cracks.” My guess is that they tested the engines, found that the cracks were not a significant problem for a single flight, especially because the Falcon 9 rocket uses nine Merlin engines on the first stage and thus has some redundancy should one fail. And based on SpaceX’s flight record — no launch failures due to failed engines — that conclusion seems reasonable.

SpaceX is now redesigning to eliminate the cracks, however, because such cracks are not acceptable for engines that will fly multiple times on reused first stages.

Thus, this story, as leaked, appears to me to be a hit job by powers in Congress who dislike the competition that SpaceX poses to big government rockets like SLS. SLS will use salvaged shuttle engines, designed initially for many reuses, and thus are superior in this manner to SpaceX’s Merlin engines. The shuttle engines however were also built by the government, which didn’t care very much about the cost of development, or making any profits. The comparison thus is somewhat bogus. Moreover, I suspect these cracks were only discovered after SpaceX successfully landed and recovered some first stages. To put them on trial in the press now for doing good engineering research and redevelopment seems somewhat inappropriate.

The report itself has not yet been released, though it does also note lingering issues with the parachutes being developed for Boeing’s Starliner capsule.

Overall, both companies are struggling to start their operational flights by 2019. For Congress or NASA to try to put more roadblocks up in that development seems most counterproductive.



  • LocalFluff

    I haven’t even seen a rocket engine in real life, but I would think that a turbopump propeller is hard to replace. That it is embedded in an environment of high pressures and lots of energy so that one wants it to be surrounded by solid metal without seams and hard to disassemble. This must be one of the most complicated parts of a rocket engine. Cavitation destroys even propellers on seagoing ships and they are regularly replaced on jet aircrafts. Imagine the extreme conditions of a rocket engine, and the complexity of the physics involved to describe what happens to it. A reusable rocket engine maybe should be designed to have it easily replaced, and maybe it is in the Merlin engine.

  • Alex

    @LocalFluff: It seems logical to me, that you are not qualified to give a technical statement about LRE here, if you haven’t even seen a rocket engine in real life up to now.

  • Tom Billings

    The Merlins currently in use are designed to be used for the expendable program. Thus, the small cracks *after* they have gone through a fast cooling/contracting regime when the engine shuts down are no big problem. The Block 5 engines are part of the redesign for full reusability. Those will be in a different situation for both testing and flight, and will be designed to reduce the cracking. Likewise, the design for the Raptor engines will be designed to go through many flight cycles.

    It is notable that Andy Pastorz is the author of this article. He has proven to be one of Senator Shelby’s best hit men for getting out articles that make commercial crew look bad, as opposed to the SLS. That is the rocket Shelby’s staff specified to keep employment up in and around Marshall Spaceflight Center.

  • LocalFluff

    @Alex Yes, that’s why I stated it to begin with. Here’s a look of one of them:

    I just use common sense and put two and two together. So much in space flight is politics that one doesn’t need a science degree to understand or predict it. It’s just about a bunch of stupid people who make a living off pretending to not being able to cooperate with each other.

  • C Cecil

    Mr. LocalFluff, Just for clarification, the component in the turbo pump that pressurizes the fuel and/or lox is called an impeller as opposed to a propeller. From what I understand, the cracking is in the turbine wheel that drives the impeller. There are many cross-section views of turbo pumps on Google images. Have fun looking.

  • LocalFluff

    @C Cecil It is the fastest moving part in the engine and it has a very extreme and a vastly shifting environment. It’s an obvious weak point. If this actually is a problem, it could be a very serious challenge for reusability. That’s my conclusion. A helium tank in the upper stage might be easily reinforced, losing only a fraction of the payload capacity. But the fuel pump is central to the whole design and even to the very business idea of SpaceX.

  • Dick Eagleson

    Over the history of the Merlin engine family, SpaceX has boosted thrust by a rough factor of 2.5. The Block 5 version of the Merlin 1-D is supposed to push this multiple very close to 3. With all the upgrades, the engine hasn’t gained much mass. The Merlin 1-D only weighs a bit over a half ton. The easiest way to make a mechanical component more robust is to, literally, beef it up. The Full Thrust version of the Merlin 1-D has a thrust-to-weight ratio on the high side of 180. That’s miles ahead of any other large liquid propellant rocket engine ever built. Given the extra thrust planned for the Block 5 version of Merlin 1-D, a 10% increase in the engine’s total mass – all of it allocated to beefier turbomachinery – would still leave the T/W ratio over 180. A 10% increase in overall engine mass would buy a much larger percentage increase in just the mass of the turbomachinery. I strongly suspect SpaceX’s next iteration of Merlin 1-D will spend its extra performance for extra mass to purchase the long-term robustness needed for serial reusability without backsliding on any of the engine’s other figures of merit.

  • Edward

    Dick Eagleson,
    Thank you for the research and analysis.

    I think that you are right that the Merlin engine will be an important workhorse for quite some time.

    SpaceX has been seen as important to the industry because it is revolutionizing launch to orbit. Blue Origing is revolutionizing the sounding rocket industry. Bigelow is revolutionizing the space station industry. ULA plans to revolutionize space travel (if only for unmanned spacecraft) with its ACES craft, and could revolutionize Lunar exploration with its Xeus craft. If Skylon is successful, it could further revolutionize access to space and leave SpaceX scrambling to catch up.

    “Launch, Land, Repeat” is Blue Origin’s famous slogan, but a variant showed up in the Smithsonian’s Air & Space magazine in May of 2005.

    For the past few decades, government projects created evolutionary changes, incremental changes that slightly improved a vehicle or a method. Much of the governmental revolutionary advancements, such as the Space Shuttle and unused hardware such as the NERVA engine, have given way to tried and true methods and equipment.

    But today we are seeing commercial industry making bold moves to dramatically change how we explore and work in space, leaving governmental agencies around the world scrambling to catch up.

    One of the reasons that I admire Peter Diamandis is that he boldly set a challenge for reusable space hardware that many people accepted and rose up to meet. He didn’t know whether anyone could do it, and he didn’t have the hefty reward money when he offered it, but envisioned it happening just because the challenge was set.

    Success begets success, and whereas a decade ago it was difficult to find investors for commercial space companies (Kistler failed to find them), they are now getting exited about investing in the future of commercial space travel and exploration.

    We have several visionaries, where space is concerned, and SpaceX has some of them.

  • Edward

    Important typo: that should read “May of 2015”

    What a difference a decade makes, in these exciting, fast-paced times.

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